A dryer known from the prior art is used for drying paint coats on motor vehicle bodies and for this purpose provides heated air in a dryer housing, which is also called a dryer tunnel. With the aid of the heated air, solvents present in the paint coat(s) can be evaporated, in order to achieve curing of the paint coat.
Additionally or alternatively, the heated air can bring about crosslinking of polymer chains in the paint coat, so as to give the latter a desired strength. Through the evaporation of the solvent, which may be water inter alia, the heated air in the dryer housing is cooled and becomes enriched with the evaporated solvent. In order to guarantee uniform heat treatment of the motor vehicle bodies and uniform drying of the paint coats, circulation of the air in the dryer housing must take place. Furthermore, heat must be supplied to the dryer housing in order to maintain the temperature of the air at a predeterminable minimum level, since the temperature drops due to evaporation processes and heating of the workpieces. In addition, the air charged with solvent must be exchanged and purified in order to keep the drying rate of the dryer constant.
In order to ensure that the air withdrawn from the dryer housing for regeneration purposes can be discharged into the environment as unpolluted as possible, in the case of the known dryer the exhaust air is drawn off from the dryer housing by means of a fan and supplied to a combustion device. In the combustion device, which in particular can take the form of a thermal post-combustion device (TPC) or a regenerative post-combustion device (RPC), an ignitable mixture with the exhaust air is formed by supplying a fuel, in particular gas, and the mixture is burnt. In the process, substantial neutralisation of the pollutants present in the exhaust air takes place. The exhaust gas, heated by the combustion process, from the combustion device is passed via a heat exchanger, which enables at least partial release of the thermal energy contained in the exhaust gas to a fresh air flow, which for its part is supplied as heated air to the dryer housing after flowing through the heat exchanger.
Additionally, a plurality of heating units can be provided along the dryer housing, each of these being intended for sectional heating of corresponding dryer housing sections. The heating units produce an ignitable mixture of ambient air and a fuel, in particular gas, and burn this mixture. The thermal energy resulting from this is released to the air circulated in the dryer housing.
In view of the large quantities of heat which have to be provided for drying paint coats on motor vehicle bodies, considerably energy costs arise during operation of such a dryer. Particularly during part-load operation of the dryer, when the combustion device optimised for a maximum exhaust air flow rate has to be operated away from an advantageous operating point, the energy costs produce a high proportion of the overall operating costs incurred for the operation of the dryer and thus high unit costs per drying part.
The present invention is directed to resolving these and other matters.